Porous honeycomb structures are used as filtering bodies for filtering particles emitted by diesel vehicles. These filtering bodies are generally made of ceramic (cordierite, silicon carbide, etc). They can be monolithic or constituted of different blocks. In the latter case, the blocks are fastened together by bonding them by means of a ceramic cement. The whole is then machined to the required section, which is generally round or oval. The filtering body can include a plurality of adjacent passages. It is inserted into a metal enclosure. Each passage is closed at one end; the exhaust gases are therefore obliged to pass through the lateral walls of the passages; thus the particles or soot are deposited in the filtering body.
After some time of use, soot accumulates in the passages of the filtering body, which increases the head loss caused by the filtering body and degrades the performance of the engine. For this reason, the filtering body must be regenerated regularly (for example every 500 kilometers).
Regeneration consists in oxidizing the soot. To this end, it is necessary to heat the soot. The temperature of the exhaust gases is of the order of 300° C. whereas the flashpoint of soot is of the order of 600° C. under normal operating conditions. It is possible to add additives to the fuel to catalyze the reaction of oxidation of the soot and reduce the flashpoint by approximately 150° C. The heating can be applied to the exhaust gases, to the upstream face of the filtering body, or directly to the soot deposited on the filtering body. Different techniques have been developed but require a great deal of energy and are very often difficult to control.
A more recent and advantageous approach consists in local heating (for example in front of the filtering body) to ignite the soot and initiate its combustion, which propagates to the whole of the filtering body via the soot. This type of technique is described in patent applications FR-A-2 771 449 and DE-A-19530749, for example.
This solution has drawbacks, in particular because the soot is unevenly distributed in the filtering body. Mainly because the exhaust pipe that feeds the filtering body is of smaller section than the latter, there is a greater flow of exhaust gas in the core of the filtering body than at its periphery. The quantity of soot deposited is thus also greater in the core of the filtering body.
Under these conditions, if combustion is initiated locally, for example on the front face of the filtering body, the soot is thoroughly consumed in the longitudinally central portion but the quantity of soot in the peripheral area is too small to transmit heat and for combustion to propagate to all of the filtering body.
This has two consequences that significantly affect the service life of the filtering body. The poor transmission of heat radially inside the filtering body creates high thermomechanical stresses between the hot core and the much cooler periphery. These stresses weaken the structure of the filtering body. Moreover, propagation of combustion of the soot being poor at the periphery, the regeneration of the filter is incomplete and the filter is less efficient after regeneration.
There therefore exists a requirement for a body for filtering particles contained in the exhaust gas of internal combustion engines, in particular diesel engines, that improves regeneration conditions.
The invention aims to satisfy this requirement.